A person whose mouth might as well be an anus because
all that comes out of it is bullshit.
The joke is that since shit comes out of an
ass, and name-calling and spreading
slander or lies are called talking shit, that means their mouth is an anus since shit-quality words are all that comes out of it.
Bob: Climate change is a hoax.
Alice: What? No it isn't! Haven't you noticed the jet stream weakening?
Bob: Also, vaccines
don't work.
Alice:
Wow, you're an assmouth. Try saying something LOGICAL next time.
Bob: Hmmphh!
Fine!